When mobile C-arm X-ray devices are deployed, it is necessary to set the position of X-ray source and X-ray receiver in such a way that the region of the body that is to be studied is represented on as large a scale as possible in the X-ray image. In other cases, it is desired to record images of an examination subject at different magnifications. In such cases, the size of the subject changes with the distance between subject and X-ray receiver.
In order to adjust the image range, the equipment cart is initially directed adjacent to the patient table and the C-arm is placed in relation to the patient table such that a smallest possible distance exists between subject and X-ray receiver. This is done in order to obtain as great an overview as possible of the overall anatomy, to localize the subject of interest, and to represent the same in the center of the image. If necessary, the subject is then increased or reduced in size in the image or image field of view. This is accomplished by an actuation of the lifting device, which in most cases takes the form of a lifting column that is often already motorized. The image field of view is in this case magnified by raising the C-arm and reduced by lowering the C-arm. In this way, a kind of zoom function is realized, this being referred to hereinafter as a “mechanical” zoom in order to distinguish it from the electronic zoom function within the recorded image.
Unlike in the case of stationary X-ray devices, (e.g., floor-mounted or ceiling-mounted X-ray devices), in which the distance between the X-ray source and the X-ray receiver is variable, the problem that arises with mobile C-arm X-ray devices, in which the distance between the X-ray source and the X-ray receiver may not be changed, is that the subject remains in the center of the image only in a strict AP (anterior/posterior) position of the C-arm, (e.g., an exactly vertical orientation of the C-arm), in which angulation angle and orbital angle are each zero, when the lifting column is raised or lowered, because the movement then takes place along the central axis. If the position (pose) of the C-arm diverges therefrom, e.g., if the C-arm is positioned at an angle (e.g., orbital/angulation angle is nonzero), it is not just the zoom, (e.g., the scaling factor), that changes as the lifting column is raised or lowered, as is desired, but also the position of the subject in the image. The central axis moves outward away from the subject. The subject wanders around in the image or even strays outside of the image.
This problem was solved in the prior art by correctively adjusting the C-arm position manually when performing enlargements with the aid of the mechanical zoom in order to re-center the subject in the image.